Wondering if anyone has done this?
Avaya MM uses similar permission in AD and Exchange to access messages and also uses the "ms-Exch-Recorded-Name" schema attributes (I assume there is no conflict since their attributes start with OCTEL not Cisco).
The customer will have both systems running during transition.
I performed a similar migration, from the precursor of MM, Unified Messenger 4.X. You are correct, both schema attributes can coexist and will not conflict. We likewise had both applications running as we moved users from Avaya to Cisco. Note: I recommend taking a look at Unity's Optional Conversation 1, which mirrors the keystrokes in the Octel conversation, unless of course you have altered the keymap in your current MM. I do recommend deleting the user from Avaya to clean out the attribute fields though before importing into Unity. I have some user training documentation we created that I could email you separately if you'd like a copy for your own customization. It provides, I think, a smooth transition for users moving from Octel to Unity (we still have 20+ Octel servers in our network). Also, do you need voicemail integration during the transition, i.e. the ability to send voice messages between voicemail applications? There are Unity networking options available you might consider.